The present invention relates to a wavelength drop-and-insert device applicable to, but not limited to, optical communication for dropping and/or inserting a part of a group of discrete wavelengths.
A prerequisite with an optical communications network is that the frequency range of light be effectively used by dropping and/or inserting, among a great number of optical frequency signals, a signal having a particular frequency. A wavelength drop-and-insert device for meeting such a requirement has been reported by D. C. Johnson et al. in a paper entitled "New design concept for a narrowband wavelength-selective optical tap and combiner", Electronics Letters, Vol. 23, No. 13, pp. 668-669, 1987. The wavelength drop-and-insert device disclosed in this paper is a Mach-Zehnder interference type device having a first 3 dB directional coupler for splitting an input optical signal into two beams which propagate through independent optical paths by the same power, and a second 3 dB directional coupler for recombining two beams. Specifically, the second 3 dB directional coupler causes the two beams propagated through the two optical paths to interfere with each other and then outputs them via a terminal other than an input terminal. A problem is, therefore, the relationship in phase between the two beams incident to the second 3 dB directional coupler from the individual optical paths is susceptible to a change in the ambient temperature as small as 0.01.degree. C. to 0.1.degree. C., resulting in the drop/insert terminals of the device replacing each other.
Further, the prior art device of the type described causes the two beams from the individual optical paths to interfere with each other at the second 3 dB directional coupler and, therefore, has to bring the states of polarization thereof into coincidence within the 3 dB directional coupler. Generally, however, the state of polarization of light propagating through a transmission path is apt to change due to a change in the double refractive index or birefringence of the transmission path which is in turn ascribable to externally derived mechanical vibrations and pressures as well as varying ambient temperature. Thus, the polarized states of the two beams in the 3 dB directional coupler will not coincide with each other. Use may be made of a polarization preserving fiber in order to stabilize the polarized state on the optical path and to thereby maintain the polarized state constant. In practice, however, it is only a truly circular fiber or the like free from double refraction that can transmit light while maintaining a desired oval polarization as distinguished from a linear polarization. Such an implementation is extremely difficult to achieve. In this connection, even if a truly circular fiber is used, it is apt to have double refractivity when subjected even to an unnoticeable temperature variation, mechanical pressure or vibration, resulting in the polarized state of light being unstable. The difficulty in transmitting a desired polarized state stably directly translates into a difficulty in insuring stable operations with a desired polarized state (i.e. desired oval polarization). Consequently, the intensity of interference of the two beams in the 3 dB directional coupler changes to bring about various undesirable occurrences such as the leakage of light to an unexpected terminal of the device.